They are designed to withstand pressure from the weight of multiple figures. I've done two Alternators-scaled combiners, and these feet support the weight easily, so you can bet it'll work just as easily with a Classics scale custom.

Begin by creating a box in box construction. Your inner box needs to fit the leg combiner figure. The outer box NEEDS to be wider and taller than the inner box to allow pivot. Find the center of both boxes and drill a hole at each end. You then bolt the two pieces together. You can adjust the tension of the bolt depending on the weight they are to hold:

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Take the part of the leg figure that will be mounted to the feet and set them in a central location. In my own build, I use the ankles of the figure as the pivot point to move forward and back. (The scratch built foot assembly gives it the side to side motion):

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For forward support, I use a heavy sheet of styrene laid on an angle over the robots feet. The angle allows for a wedge so the feet can be pulled back and out of the area:

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To keep the leg robots feet from slipping out, we need to build a spring loaded piston using styrene tubing and a nice sized spring:

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Here's the assembly in place. Be sure to notch out the piston sleeve and put a screw in there to keep the spring from popping out when not in use. To release the figure, just pull back on the screw head and remove it:

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You can now begin scratchbuilding your foot details and overall design. (This would be built on the inner box, not the outer):

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In the event you're using grossly off scale figures. You have a few options. You can mount the connection point in a lower area and compensate with your central combiner robot as well as . . . .

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. . . . build that particular foot plate to lift the figure higher on that side. The yellow footplate has an equally sized silver "shoe", but the yellow platform is taller. The Jeep is also mounted higher in the foot whereas the Tank is recessed. The yellow foot also needs to be wider so a ball-jointed side skirt system is constructed:

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. . . . and it pops out like this:

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This is the articulation you get out of this design:

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If you plan to enclose your combiner feet into something useful that is transformable itself, read on:

In this example, the basic box-in-box foot construction is glued into a section of a semi’s trailer. Now you need to fit the complete figure in place and fill in your details around it. If you're building a combiner in the alternators class, I would recommend another support structure be utilized for the forward and back motion. Ball joints can only take so much weight before they pop out, by using a setup as seen here, the weight gets distributed across the whole foot rather than just on the figure. For a classics scale, the support system is not necessary but can add detail:

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You can attach this platform to whatever you want or style it anyway you want. In the case of this one, the foot becomes a section of a semi’s trailer. At this point, it's time to scale up your second leg. It's important to have them close to the same height when combined or one leg will always be walking downhill. Using this way of building feet, you can also offset a difference in height by building a deeper or shallower foot cradle:

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As you can see, the side to side motion is all in the foot. The forward and back movement is all in the figure:

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Here it is transformed into its compact form. You can build it any way you like. Your imagination is the limit. Oh, and the size of the component it's going into:

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Always, always test fit. And after you've test fitted everything. Test fit it again. You can go a long way only to have to go back and redo everything: